A method for correcting pattern critical dimension (CD) in a photomask includes forming a multilayer structure over a substrate by stacking at least two thin films capable of forming a compound by application of energy from an energy source; forming a light-shielding layer over the multilayer structure; forming a light-shielding layer pattern that selectively exposes the multilayer structure by selectively etching the light-shielding layer; detecting a correction region requiring a CD correction by measuring a CD of the light-shielding layer pattern; and forming a compound, by which the CD is corrected by a transmittance difference between the multilayer structure and the correction region, by applying an energy to a region of the multilayer structure corresponding to the detected correction region to react the thin films.